1. Field of the Invention
The present invention relates to a rubber composition which can sufficiently improve performance on ice and snow and wet gripping properties, and a pneumatic tire having a tread made of the rubber composition.
2. Description of the Related Art
Heretofore, when automobiles are driven in winter in a cold region with heavy snowfall, studded tires with spikes are used or chains are wound around a tire to ensure safety on snowy and icy road. However, studded tires or chain-equipped tires easily cause abrasion and damage of the road surface and dust is produced therefrom, resulting in air pollution and this was a significant environmental problem.
As a result, in order to solve the problems of safety and environment studless tires which exhibit certain braking performance and steering properties on ice and snow without spikes and chains have been rapidly developed.
As a studless tire, automobile tires having improved performance on ice by compounding sand (JP-A-61-150803) or metallic fibers (JP-A-63-34026) into the tread rubber are suggested. However, in such tires, hardness of the rubber becomes relatively high, and therefore friction on ice is insufficient. In addition, as tire wears off, those sand, metallic fibers and other metals fall off and the particles are spattered to cause a serious environmental problem of dust pollution.
Alternatively, using foamed rubber for the tread rubber is proposed (JP-A-62-283001, JP-A-63-9042 and JP-A-1-118542). However, in these tires, though frictional force on ice and snow is improved, edge effect and drainage effect from closed cells cannot be fully utilized due to low block stiffness of the foamed rubber and also, abrasion resistance and steering properties on dry road are inferior. Moreover, since foaming is conducted in the vulcanization step of the production, dimensional accuracy tends to be poor.
Also, techniques of improving tire performance on ice and snow by compounding hollow particles in the tread are proposed (JP-A-11-35736 and JP-A-6-328906). However, in these tires, fine hollow particles break when kneading, resulting in the problem that sufficient performance on ice and snow cannot be obtained.
Furthermore, techniques of improving performance on ice and snow by compounding a water-absorbing synthetic polymer in the tread rubber and removing water existing between the road surface and the tread are proposed (JP-A-5-148390). However, the water absorbing ability of synthetic polymer is insufficient and also the edge effect after falling of polymer is insufficient because polymer is difficult to fall immediately after water absorption.
In addition, a method of improving gripping properties on ice by increasing digging friction by compounding and orientating short fiber in the direction perpendicular to the tread surface is proposed (JP-A-2000-168315). According to these techniques, the gripping properties on ice of studless tires improved but not yet comparable to that of studded tires.
Also in a region which has melting of snow, wet gripping properties are also required for studless tires but in the conventional method of preparing a studless tire, gripping properties on wet roads of rainwater and snow melting water are insufficient.